1. Field of the Invention:
The present invention relates to a positive let-off motion for a loom and, more particularly, relates to a warp tension control method for accurately controlling warp tension in synchronism with the principal motion of the loom so that warp tension coincides with a target value.
2. Description of the Prior Art
A let-off motion for a loom regulates warp yarn feed rate according to the positional variation of a tension roller over which warp yarns are passed. The warp tension is dependent on a force applied to one end of a tension lever supporting the tension roller at the other end thereof by a mechanical means such as a weight block or a tension spring. Accordingly, a target warp tension can mechanically be determined in a range by using a weight block having an appropriate weight or by using a tension spring having an appropriate spring constant.
Incidentally, the warp tension varies pulsatively in synchronism with the principal motion of the loom during each revolution of the main shaft. Therefore, it is necessary to correct the warp tension properly when the warp tension is increased temporarily by the shedding motion or beating motion of the loom or when the tension of warp yarns on a towel loom needs to be reduced to form warp piles.
An easing motion is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 59-133687.This easing motion changes the position of a tension roller positively for warp tension to relax the warp tension.
However, in such an easing motion, a corrected warp tension does not necessarily coincide with a target warp tension because the target warp tension is defined by a displacement of the tension roller. That is, since the relation between the actual warp tension and the displacement of the tension roller is dependent on the Young's modulus of the warps, weaving conditions and actual weaving circumstances, and the actual warp tension is not always exactly proportional to the displacement of the tension roller, such a known easing motion is unable to adjust the actual warp tension correctly at a target tension even if the tension roller is displaced by a predetermined displacement. Furthermore, when an occasional warp tension control is required to relax the warp tension only once every several turns of the main shaft of the loom, for example, in forming piles on a pile fabric loom, accurate pile forming operation is impossible because the tension of the pile warp yarns varies more frequently.
Thus, although this prior easing motion may be able to suppress the temporary rise of the warp tension, the easing motion cannot achieve accurate warp tension control operation by any possibility because the controlled variable is not the warp tension, but is the displacement of the tension roller, which is different in dimension from warp tension.